1. Field of the Invention
The present invention relates to an optical fiber coupler, and more particularly to a single-mode optical fiber coupler in which biconical optical fibers are coupled with each other.
2. Description of the Related Art
In a conventional optical fiber coupler, longitudinal coatings of the same kind of optical fibers are partly removed and exposed glass fiber portions of the optical fibers are fused with each other and elongated at the fused portion in order to accomplish optical branching and coupling. Generally, multi-mode optical fibers are used.
However, as seen in the paper by Morishita entitled "Design of Optical Fibers for Wave-length Selective Melt-type Optical Couplers", in Extended Abstracts, The Spring Meeting, 1989, the Institute of Electronics, Information and Communication Engineers of Japan, No. 4- Pages 427, an optical coupler for combining and branching light at a specified wavelength is being studied. In the optical coupler proposed in the above paper, optical fibers to be used are of different core diameters and have a difference in refractive indices between the core and the cladding of each optical fiber so as to combine and branch light of 1.53 .mu.m and 0.83 .mu.m. That is, single-mode optical fibers, having wavelengths which are comparatively far from each other, are used to thereby produce coupling of light in a cladding mode at a long wavelength by utilizing the fact that the optical fibers greatly differ from each other in distribution of light propagated. It is therefore impossible to perform combining and branching of light with wavelength selectivity at wavelengths close to each other.